1. Field of the Invention
This invention relates generally to optical fibers both graded index multi-mode and single-mode, and particularly to a method of monitoring both the modal phase delay and power of a microwave signal which is transmitted by the fiber as optical radiation. This method of monitoring enables effective detection of intrusions into secure fiber optic communication links.
2. Description of Related Art
The development of graded index fibers was an important advance in optical fiber technology. In a graded index fiber, the refractive index varies from a maximum axial value, radially throughout the core, but is usually constant in the cladding region. The index distribution, that is, variation of the refractive index n as a function of the radius, of a graded index fiber may be written as:       n    ⁡          (      r      )        =      {                                                                                                            n                    1                                    ⁡                                      [                                          1                      -                                              2                        ⁢                                                                              (                                                          r                              /                              a                                                        )                                                    g                                                ⁢                        Δ                                                              ]                                                                    1                  /                  2                                                                                                      for                  ⁢                                      xe2x80x83                                    ⁢                  r                                 less than                 a                                                                                        n                o                                                                                      for                  ⁢                                      xe2x80x83                                    ⁢                  r                                 greater than                 a                                                    ⁢                  xe2x80x83                ⁢        where            ,              
            ⁢                                                                  a                =                                ⁢                                  core                  ⁢                                      xe2x80x83                                    ⁢                  radius                                            ;                                                                          r              =                            ⁢                              radical                ⁢                                  xe2x80x83                                ⁢                coordinate                                                                                                                          n                  1                                =                                ⁢                                  refr                  ⁢                                      xe2x80x83                                    ⁢                  active                  ⁢                                      xe2x80x83                                    ⁢                  index                  ⁢                                      xe2x80x83                                    ⁢                  on                  ⁢                                      xe2x80x83                                    ⁢                  the                  ⁢                                      xe2x80x83                                    ⁢                  axis                                            ;                                                                                                            n                  o                                =                                ⁢                                  refractive                  ⁢                                      xe2x80x83                                    ⁢                  index                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  the                  ⁢                                      xe2x80x83                                    ⁢                  cladding                                            ;                                                                          Δ              =                            ⁢                              a                ⁢                                  xe2x80x83                                ⁢                constant                ⁢                                  xe2x80x83                                ⁢                which                ⁢                                  xe2x80x83                                ⁢                is                ⁢                                  xe2x80x83                                ⁢                given                ⁢                                  xe2x80x83                                ⁢                by                ⁢                                  xe2x80x83                                ⁢                the                                                                                                      ⁢                                                relative                  ⁢                                      -                                    ⁢                  index                  ⁢                                      xe2x80x83                                    ⁢                  difference                                ,                                  xe2x80x83                                ⁢                namely                ,                                                                                                      ⁢                                                                                                                  n                        1                        2                                            -                                              n                        o                        2                                                                                    2                      ⁢                                              n                        1                        2                                                                              ≈                                                                                    n                        1                                            -                                              n                        o                                                                                    n                      1                                                                      ;                                  xe2x80x83                                ⁢                and                                                                                        g              =                            ⁢                              exponent                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                the                ⁢                                  xe2x80x83                                ⁢                power                ⁢                                  xe2x80x83                                ⁢                                  law                  .                                                                        
(See Optical Fiber Telecommunications, Eds. Stewart E. Miller and Alan G. Chynoweth, Academic Press, 1979, Ch. 3 xe2x80x9cGuiding Properties of Fibersxe2x80x9d, pp. 38). In the special case when g=2, the fiber is said to be a xe2x80x9cparabolic-indexxe2x80x9d or xe2x80x9csquarelawxe2x80x9d or xe2x80x9coptimum power lawxe2x80x9d fiber.
As mentioned earlier, the core of a graded index fiber has an index of refraction that varies radially from the axis outward to the periphery. Ideally, this gradual variation in refractive index should be such that it exactly compensates for the variation in path lengths of the different modes of propagation of optical energy along the fiber, so that all the modes travel along the fiber at the same axial velocity. In actuality, although such perfect compensation cannot be achieved, modal dispersion can be greatly minimized.
Fiber characterization by accurate measurement of important parameters such as attentuation, bandwidth, modal delay distortion, pulse dispersion, refractive-index profile and mechanical strength of the fiber provides the data necessary both for systems design and for development of better fibers. The increasing use of fiber optic communication links has been accompanied by the effort to develop higher bandwidth fibers, and many techniques have been developed for convenient and accurate measurement of the bandwidth and other parameters of fibers needed to provide the data necessary for this development effort.
The bandwidth of transmitting fibers is limited by dispersion, whereby a narrow or rectangular pulse of optical energy is spread out or smeared in time as it travels along the fiber. The effect of such pulse dispersion is to limit the potential bandwidth of the transmitting fiber. This dispersion, referred to sometimes as xe2x80x98delay distortionxe2x80x99, can be measured by one of several techniques, either in the time domain (impulse-response measurements) or in the frequency domain (transfer-function measurements). See the article by Michael K. Barnoski and S. D. Personick xe2x80x9cMeasurements in Fiber Opticsxe2x80x9d in Proceedings of the IEEE, vol. 66, no. 4, pp. 436-8, April 1978, for a review of the above-mentioned techniques and a brief discussion of the causes and effects of delay distortion. Briefly, the time-domain, impulse-response measurement approach involves injecting a narrow pulse of light into one end of a fiber, detecting the broadened output pulse at the other end, and determining the time delay for different modes. The frequency domain phase delay measurement technique uses frequency modulation of the input optical beam.
The ultimate goal of the fiber characterization techniques briefly discussed above is generally only the selection of a fiber which minimizes phase and time delay distortion during transmission. However, an important and additional goal of systems for detecting intrusions into secure communication links is the selection of a fiber which both minimizes delay distortion and has maximum sensitivity to intrusions.
Conventional secure optical communication systems use masking signals and alarm arrangements to make the data-carrying signal secure. A masking signal is launched, at an input angle different from that of the data-carrying signal, into the communication line. This prevents intruders from separating the data-carrying signal from the masking signal when they observe the line radially. Limitations of such arrangements include low bandwidth of the order of 10 to 20 Mb/sec, and applicability only to point-to-point communications.
It is well-known that when a transmitting fiber is perturbed by an intruder, changes in the transmitted power occur. Therefore, intrusion detection systems have been implemented wherein power monitoring of a guard signal is used. Attempted access to the inner information-carrying core causes a decrease in intensity of a security signal being transmitted in an outer concentric core, alerting the operator to the attempted intrusion. However, a major drawback of such systems is the difficulty of maintaining power stability, that is, the difficulty of avoiding fluctuations in the output power caused not by intruders but by the optical components themselves.
Consequently, there is a need for an intrusion detection system which can be used in point-to-point and network applications, which works with graded- and single-mode fibers achieving high bandwidth, and has a high degree of accuracy and sensitivity.
An optical fiber for secure transmission of an optical beam therethrough and for enhanced sensitivity to intrusion, is described. The fiber comprises:
a cladding, and
a primary core having a cross-sectional profile, said index profile having at least first, second and third regions having a graded index, a first step index region interposed between said first and second regions, and a second step index region interposed between said second and third regions,
said index profile being uniform along the length of said fiber,
said index profile for enhancing modal phase delay variation in said beam responsively to said intrusion.
It is an object of the present invention to provide a fiber optic system for detecting and signalling intrusions into fiber optic communication links.
It is another object of this invention to provide a system for and a method of simultaneously making modal phase delay and power measurements whereby intrusions into secure fiber optic communication links can be detected.
It is a further object of this invention to provide a system for testing optical fibers for suitability for use in intrusion detection systems.